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Monitoring Near-Surface Soil Water Storage in Turfgrass using Time Domain Reflectometry and Weighing Lysimetry

Dep. of Soil, Water and Environmental Science, Univ. of Arizona, Tucson, AZ 85721

C. F. Mancino

Dep. of Agronomy, The Pennsylvania State Univ., University Park, PA 16802

*Corresponding author (youngmh{at}ag.arizona.edu).

ABSTRACT

Monitoring near-surface soil water content with time domain reflectometry (TDR) could improve our understanding of water movement in a daily irrigated crop. But, TDR field validation is needed using weighing lysimeters to assess actual changes in soil water storage. In this study, we test the hypothesis that vertically installed TDR probes can be used to measure evapotranspiration (ET) in a full-cover turfgrass environment. This was done by directly comparing changes in soil water storage measured with TDR against changes in mass measured with weighing lysimeters. The TDR probes (200, 400, 600, and 800 mm long) were installed vertically in two weighing lysimeters. The TDR water content and lysimeter mass determinations were made every 30 min for a 28-d period during daily irrigation intervals, and during a 6-d irrigation interval. Average water contents increased with increasing probe length; e.g., 600- and 800-mm probes yielded average water contents of 18.1 and 18.5%, vs. 14.8% for the 200-mm probe. These differences reflect the lack of transpirational uptake from deeper soil layers. All TDR probes underestimated water added and lost compared with the lysimeter during daily irrigation, with shorter probes exhibiting larger differences than longer probes. However, when the irrigation interval was increased to 6 d, the longer TDR probes were more suited to measuring water loss; for example, the 200-, 400-, 600-, and 800-mm probes measured 40, 77, 78, and 96% of the water loss recorded by the lysimeter, which only records ET. This shows that increasing amounts of water were taken up from deeper layers as the soil dried.

Received for publication June 25, 1996.

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